dmix: Don't use assert() and abort()
dmix: Don't use assert() and abort()

We seem to still have some races at closing a dmix stream, but
aborting is the worst option.  Let's make not melt down.

Reference: https://bugzilla.novell.com/show_bug.cgi?id=852446
Signed-off-by: Takashi Iwai <4596b3305151c7ee743192a95d394341e3d3b644@suse.de>

tiwai/alsa-lib,alsa-project/alsa-lib,mikelima/alsa-lib,mikelima/alsa-lib,mikelima/alsa-lib,mengdonglin/alsa-lib,Distrotech/alsa-lib,takaswie/alsa-lib,lgirdwood/alsa-lib,lgirdwood/alsa-lib,tiwai/alsa-lib,zedongchen/alsa-lib,takaswie/alsa-lib,lgirdwood/alsa-lib,alsa-project/alsa-lib,mikelima/alsa-lib,mikelima/alsa-lib,tiwai/alsa-lib,takaswie/alsa-lib,lgirdwood/alsa-lib,zedongchen/alsa-lib,takaswie/alsa-lib,alsa-project/alsa-lib,tiwai/alsa-lib,takaswie/alsa-lib,zedongchen/alsa-lib,lgirdwood/alsa-lib,mengdonglin/alsa-lib,mengdonglin/alsa-lib,Distrotech/alsa-lib,zedongchen/alsa-lib,mengdonglin/alsa-lib,mengdonglin/alsa-lib,alsa-project/alsa-lib,Distrotech/alsa-lib,Distrotech/alsa-lib,Distrotech/alsa-lib,zedongchen/alsa-lib